Interface circuits are known that are able both to receive an input signal via the input terminal and to produce an output signal via the input terminal by controlling the current uptake of the current sink.
Such an interface circuit is described, for example, in the data sheet VNB35NV04/VNP35NV04/VNV35NV04/VNW35NV04 “OMNIFET II”, ST Microelectronics, March 2004, page 5/19. The interface circuit described in this publication is part of an actuator circuit of a MOSFET, which receives an actuation signal for the MOSFET via an input terminal. If the MOSFET has an excess temperature, the interface circuit draws an elevated current via the input terminal. This elevated current uptake at the input terminal is detected by an external diagnostic circuit connected to the input terminal as a sign of the presence of an excess temperature.
The elevated current uptake stresses the signal source which furnishes the input signal. If the signal source has a high output resistance, the elevated current uptake of the interface circuit may result in the signal source no longer being able to maintain the input signal level. Thus, the signal level drops, which is misinterpreted in the interface circuit as being a change in the signal level of the input signal.